AT&T is one of several major firms requiring office workers to work on-site five days a week.Kena Betancur/VIEWpress/Getty ImagesAT&T began implementing its staggered five-day return-to-office mandate on Monday.Workers told BI that limited available desks and elevators at some locations complicated their office return.As more workers are slated to arrive in phases, projects to add capacity are underway.The first wave of AT&T's five-day return-to-office mandate started on Monday.